Lens for motion-picture cameras



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Dec. 26, 1950 F. G. BACK r-:rAL

mus Fox lonen-PICTURE cmms Filed Oct. 14. 1948 :Fi CF1..

Patented Dee. 26, 1950 LENS FOR MOTION-PICTURE CAMERAS Frank G. Back, New York, and Herbert Lowen, Forest Hills, N. Y.; said Lowen assignor to said Back Application October 14, 1948, Serial No. 54,482

1 Claim.

This invention relates to new and useful improvements in lenses and has especial reference to those adapted for use in motion picture cameras.

An object of the invention is to provide a lens unit which is substantially free from aberrations, such as distortion, spherical aberrations, curvature of field, and coma.

A further object is to provide such a lens as can be stopped down without shifting the image plane.

A further object is to provide a very fast lens with a relative aperture of F:1.3 with a completely satisfactory resolution.

A further object is to provide a lens which is practically free from primary and secondary color, laterally and longitudinally.

Still another object is to provide such a lens that can be focused for close objects by simply moving the front lens unit instead of the whole lens.

Yet another object is to provide a combination of lens housing and supporting tubes so mechanically related that they can be rotatably adjusted relative to each other to a very flne degree without the useof screw threads.

Still another object is to provide a simple means whereby the tubular housing may be very quickly and easily adjusted in relation to the tube housing the rear lens combination to enable the operator to readily see the reference marks on the graduated scales and to t the housing snugly into place against the tube housing the rear lens combinations, when so readjusted.

Further and more speclilc objects, features and advantages will more clearly appear from a consideration of the detailed specification hereinafter set forth, especially when taken in connection with the accompanying drawing which illustrates a present preferred form which the invention may assume and which forms part of the specification.

In brief and general terms the invention concerns -the provision of a series of positive doublets, three in number, achromatic in nature, in a lens tube, one doublet in the front thereof and adjustable, and of relatively lower power, and two in the rear of the tube, spaced closely to each other, and of relatively larger power.

:,.The distance between the front doublet and the 'nearest rear doublet is relatively large relative tothe space between the respective rear doublets. Each doublet is composed of a positive crown abutting a negative flint. In the lens all the crowns face one way and the ilints the other way.

In another particular, the invention concerns the provision of a resilient frictional contact between the lens housing and a lens supporting tube rotatable thereon to permit fine adjustments of the tube on the housing and dispensing entirely with the use of screw threads. Preferably this takes the form of a resilient finger cut out of one of the elements and frlctionally engaging the adjacent surface of the other.

Still further, the invention includes the provision of a bevel or inclined annular slope near the end of the lens housing into which bevel screws on the lens tube may project whereby as the pin or screw are projected into the bevel, the slope of its bottom will axially position the tube on the housing. Y The present preferred form which the invention may assume is illustrated in the drawings, of which- Figure 1 is a side elevation of the lens unit;

Fig. 2 is an enlarged sectional view through the unit; and

Fig. 3 is a cross sectional view taken on the line 3-3 of Fig. 2.

Referring now merely to the specinc form of the invention shown in the drawings, it is seen that the device includes six lens elements i. I, 3, 4, 5 and 6. Elements I, 3 and S are positive crown lenses and elements 2, I and t are negative flint lenses. Each crown is cemented to a flint as indicated in such a way as to form achromatic doublets. These doublets are positioned in such a way that all the positive crown elements face the object while the flint negative elements face the film.

Preferably for the crown elements we use a low dispersion glass (with a high v) and for the ints we employ a dense flint with a high dispersion (low v). In the example hereafter in detail set forth we combine borosilicate crown glass (BSC-2) with a dense barium flint glass (DBF-1). In this manner we can make the crowns of the rear lenses symmetrical. The front doublet comprising lenses I and 2 has only :V4 the power of each of the two rear doublets.. The distance between the front doublet formed by lenses l and 2 and that formed by lenses I and l is quite substantial compared to the short distance between the latter doublet and the one formed by lenses 5 and t as shown. The greater distance is indicated by the arrowed line in Fig. 2 along the horizontal center line of the ngi-1re.

4spring finger.

The structure includes a tubular housing 1 with the iris or diaphragm 8 disposed as shown somewhat nearer the rear doublets than the front doublet. The rear doublets are housed in a tube 9 having a flange III encircling the tube 1. The front doublet I and 2 is housed in a tube II to which is tightly tted a sleeve I2 rotatable on the main tube 1 with a pin I3 therein riding in a curved slot I4 in the tube 1 to adjust the front doublet axially in focus. A ring 20 is disposed on the tubular housing 1 between the sleeve I2 and the tube 9. It is, of course, understood that the exact position of the diaphragm and the exact shape or curvature of the lenses is a matter of correction well known in the art and dependent upon the results desired.

In order to facilitate the rotary adjustment of the sleeve I2 and ring 20 on the tubular housing 1, we have dispensed with the use of the usual expensive screw threads which are easily damaged and provide a resilient frictional contact between them and the housing element in the form of a In Figs. 2 and 3. this spring i'lnger I5 is shown cut out on the cylindrical surface of the housing 1 with a very narrow curved slot IB between it and the adjacent body of the tube 1.'

It is, of course. obvious that this spring finger will press resiliently against the adjacent inner surface of the ring 20 to frictlonally hold the ring 20 in any adjusted position. The pin and vslot action I3 and I 4 control the axial movement of sleeve I2 and the spring finger I5 controls the circumferential movement. It is found that very fine adjustments can be made without the expense of screw threads which are constantly being damaged. At the left in Fig. 2 is shown a similar spring finger formed to press against the inner surface of the sleeve I2 and this is identical, except for size, with the one shown in connection with the ring 20.

The invention further relates to a simple means for permitting the operator to adjust the ring 20 on the housing 1. By reason of the fact that different cameras have variable starting points on their threads at the place where the lens is .screwed on, it sometimes happens that the reference marks indicated by the numeral I1 in Fig. l are not visible to the operator so that it becomes necessary to turn the housing 1 around so that the operator can see the reference marks. To permit this, the screw IB in flange I0 which ordinarily projects from it into an annular bevel I9 on the adjacent surface of housing 1 is loosened to permit the housing to be turned to bring the reference marks I1 into the view of the operator, thereafter the pin or screw I8 is moved into place with its inner end projected forcibly against the slope of the annular bevel I9 in the housing 1, said annular bevel sloping downwardly from the end of the housing toward the middle thereof. It is thus that the forcible projection of the pin or screw I8 against this slope of the annular bevel will tend to move the housing 1 axially so that it will be firmly pressed against the flange I D of tube 9. This will insure a secure definite position for the tube 9 with its supported lens doublets.

In order to adjust the diaphragm 8, the tubular housing 1 is provided with a circumferential slot 2| through which projects a screw threaded pin 22 disposed on the ring 20 so that as the ring 20 is turned on the housing 1 the diaphragm 8 connected to pin 22 may be adjusted.

As an example the following is a specification of one such a lens combination as we have described:

Thicknel Lans Glass Radii and Separation L1 BSC-2 Erri-95.76 T1-l7.56

Nn=l.517 12g-+8104 u=64.5

L; DBF-l R187.04 Sl-O Nn=l.6l7 R4 plane T1128() r=38.5

S2162 La BSC-2 R51-HNA() Sri-25.16 Nrw-1.517 R|-+69.40 Tal8.2l I7=64.5

SI-O L4 DBF-1 E1n-69.40

Nn -l.617 Rx- 977.39 TI-5.3) v=38.5

5.5-2.6() L; BSC-2 Iig-4459.40

ND=1.517 R1-+69.40 T;-l8.2l v=64.5

S0 La DBF-1 R11- 69.40

ND=.1.617 Riz- 977.39 'rl-5.@ n=38.5

Equivalent focal length equals mm. Radii, thickness and separation in mm. In the above table the following terms may be defined as follows: R equals radius of curvature. T equals thickness.

S equals axial spacing. N equals index oI refraction.

It is to be noted that other telephoto lenses on the market consist basically of a positive front element and a negative rear element. These suffer from heavy aberrations, especially distortion, zonal spherical aberration, and coma. Our new lens consists only of positive units and'is practically free of spherical aberration and coma not only for the marginal ray but also the intermediate zones, so that the lens can be stopped down without shifting the image plane. Our lens can also be opened to a relative aperture of F:1.3 and its resolution is still completely satisfactory. It is absolutely free of distortion, and the negative astigmatism which has to be introduced to obtain a fiat field can be reduced, by methods well known in the art, to such an extent that it does not impair the image quality. As far as color correction is concerned, though it is not an apochromat in the technical asense of the Word, it is practically free not only" from primary but also from secondary color, laterally and longitudinally.

Another feature of advantage resident -in our lens consists in the fact that focusing for close objects is done simply by moving the front lens. 1

This simple focusing has the advantage that the relative aperture does not change whenthe lens is focused for a close-up. Most ordinary lenses, which do the focusing by moving the whole lens, lose speed even if the stop lever is Vnot moved. The mechanical movement in this case is several times greater than if the whole assembly is moved for focusing, and will give greater accuracy and less wear.

While the invention has been described in detail and shown with respect to the accompanying drawing, it is not to be limitedto such .details, since many changes and modifications may be made in the invention without departing from the spirit and scope thereof. Hence, it is desired to cover any and all forms and modifications of the invention which may vcome within the language and scope of4 the appended claim.

the specifications of the glasses forming the sevv eral doublets, considered from the front to the rear of the lens, being tabulated as follows:

Thickness Lens Glass Radii and Separation L; BSC-2 R|+95J6 T1sl7.56

L, DBF-l Rall-87.04 Sl-O NDIL617 R4 plane T,=2.60 III-'58.5 S1162 L. BSC-2 ils-+6940 S33-25.16 N13-1.517 R|=+69.40 '15:18.21 v-64.5

L4 DBF-1 fhg-69.40 S48() Np= 1.617 Rx= 977.39 T4I5.20 v=38.5

Ll BSC-2 Rs=+69.40 S15-2.60

ND=1.517 R-+69A0 Ts=l8.21 v=64.5

L. DBF-1 Rn 69.40 Sg-U Nr; 15617 Rn* 977.39 T|=5.20 vin which certain ofthe above terms are defined as follows:

R equals radius of curvature.

T equals thickness.

S equals axial spacing.

N equals index of refraction.

u equals Abbes dispersion number (a lass characteristic well known in the art oi optics and glass makin BSC equals borosilicate crown.

DBF equals dense barium flint.

FRANK G. BACK. HERBERT LOWEN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 943,105 Wadersleb Dec. 14, 1909 1,688,441 Stewart Oct. 23, 1928 1,883,673 Fouquet Oct. 18, 1932 1,922,537 Fouquet Aug. l5, 1933 1,988,390 Naumann Jan. 15, 1935 2,042,815 White June 2, 1936 2,321,973 Bennett June 15, 1943 2,382,669 Schade Aug. 14, 1945 

